The present invention relates to digitally recording and reproducing a television signal and, more particularly, to digitally recording and reproducing a PAL plus television signal.
Television signals are broadcast using various transmission standards. PAL is one such standard primarily employed in European countries. PAL plus is the next generation broadcasting standard of PAL that provides higher image quality than its predecessor. Another distinctive feature of the PAL plus standard is that the television picture has an aspect ratio of 9:16, which is considered more aesthetically pleasing than the aspect ratio of 3:4 which is the aspect ratio primarily used heretofore by television receivers. As shown in FIG. 27A, the PAL plus image of FIG. 27B is superimposed on the 3:4 aspect ratio of the conventional television receiver, image demonstrating the differences in aesthetic appearance between the two standards. The PAL plus standard is compatible with PAL television receivers and thus, the PAL plus television signal is derived from the PAL television signal. To obtain the 9:16 aspect ratio, a vertical conversion circuit decimates the PAL plus television signal using a 3 to 4 decimation process.
However, the decimation process deteriorates the vertical resolution of the
plus image. To correct this vertical deterioration the PAL plus standard transmits a vertical resolution signal (hereinafter referred to as a helper signal) during the upper and lower invalid screen portions (FIG. 28) of the PAL plus television signal. The helper signal is employed by the PAL plus television receiver to reconstruct the lost vertical resolution.
A problem arises, however, when the PAL plus signal is recorded digitally as, for example, occurs in a discrete cosine transform DCT process. In the DCT process, each television frame is divided into blocks of pixels (8.times.8, for example), and each block is DCT transformed. As shown in FIG. 31, the 8.times.8 DCT blocks extend into the upper invalid screen portion where the helper signal is stored. Consequently, portions of the helper signal are processed with the main screen portion in the same DCT block. Because the signal values between the DCT blocks and the helper signal are significantly different, the DCT process calculates erroneous DCT coefficients and a resultant image distortion occurs. In this case, the invalid screen portions include pixel information from the middle of the television picture, and the resultant distortion affects the center of the picture as shown in FIG. 32.